Sahih Muslim Book 6, Hadith Number 2511.
Chapter : Fasting on the day of ‘Ashura (1Oth of Muharram).
This Hadith has been narrated from Jarir on the authority of A’mash with the same chain of transmitters and he said (these words with a little bit of variation from the previous Hadith): When (fasting) in Ramadan was (made) obligatory, he abandoned it (the practice of observing fast on Ashura).